1. Technical Field
The present invention relates generally to wireless digital communications systems and in particular to wireless digital communications systems employing code division multiple access. Still more particularly, the present invention relates to defining cell coverage as a function of capacity within a code division multiple access wireless digital communications network.
2. Description of the Related Art
Digital telecommunications networks are increasingly based on code division multiple access (CDMA), a form of spread spectrum multiple access. In contrast to frequency division multiple access (FDMA), in which the available spectrum is allocated among multiple users by frequency, and time division multiple access (TDMA), in which the available spectrum is allocated among multiple users by time, users all share a common spectral frequency allocation over the time that they are active. Each active signal within a CDMA system occupies all of the bandwidth all of the time. Separation of the signals depends on each being carried by an underlying waveform which is nearly orthogonal to all other signals. Digital codes are employed in CDMA to form the underlying quasi-orthogonal waveforms. Each user within a CDMA systems has a unique long period digital sequence called a pseudorandom sequence, which appears random to a casual observation of insufficient duration. This sequence is usually either (1) combined directly with the information stream to be sent, as in direct sequence transmission, or (2) used to select pre-planned spectrum channels among which the transmission is hopped, as in frequency hopping transmission.
CDMA provides a degree of protection against frequency-selective fading and reduces multipath interference. Signals which arrive late at the receiver do not match the portion of the code currently being utilized to decode the signal, and are thus rejected as interference. CDMA is also excellent in discriminating against noise. However, all signals within a given spectrum other than the signal of interest are treated as noise by a CDMA unit. Therefore, as more users attempt to communicate via a particular spectrum, performance for all users of that spectrum gradually degrades. The capacity of a CDMA cell is thus defined based on the level of perceived noise from other users above a background noise level of mainly thermal origin.
In CDMA systems, the coverage of a cell depends on the number of users in the cell. The reason is that as the number of users in a cell increases, the ability of a given mobile unit in the cell to maintain an acceptable link with its base station decreases. Therefore, an accurate prediction of cell coverage--the size and boundaries of a cell--as a function of cell capacity is essential in CDMA network design and deployment. It would be desirable, therefore, to derive a relationship between cell coverage and cell capacity for accurate calculation of the probability of call outage. It would further be advantageous to separate the problems of call outage and call blocking in deriving a relationship between coverage and capacity, relating coverage to an average number of users for a given upper bound on outage probability to allow prediction of coverage for a projected capacity, irrespective of the admission policy utilized to achieve that capacity or the resulting call blocking probability. It would further be desirable to precisely define the limit of the average number of users a cell may support as the cell coverage shrinks to zero. It would further be advantageous to derive a relationship between coverage and capacity as described above without consideration of the effects of soft handoff.